Coating method for use in a process for producing high-temperature superconductive strip conductors

ABSTRACT

Coatings especially for superconductive strip conductors, as well as methods of applying the coatings, are addressed. Exemplary coatings include aluminum oxide in an aqueous suspension, with aluminum oxide particles contained in the suspension having size typically between 0.5-10 μm.

The present invention relates to a coating method for use in a processfor producing high-temperature superconductive strip conductors, whichare required for high-temperature superconductor cables.

From the prior art,

1) IEEE Trans. Appl. Supercond. Vol. 5, No. 2 (1995), pp. 953-955,

2) Funjikamik et al. “5m/2 KA High-TC-Cable Conductor,” Adv. inSuperconductivity V, pp. 1251-1254, Springer-Verlag 1993 (Transactionsof the 5th International Symposium on Superconductivity, 1992),

3) Sato et al. “Electromagnetic Properties and Structures of BiPbSrCaCuOSuperconducting Wires,” Advances in Superconductivity II, pp. 335-340,Springer-Verlag, 1990 (Transactions 2nd Int. Symp. on Superconductivity,1989),

high-temperature superconductor cables that are composed of a pluralityof strip conductors are known. These strip conductors, in turn, are madeof a known high-temperature superconducting ceramic material, e.g.,(Bi,Pb)₂Sr₂Ca₂CU₃O_(X), which is preferably contained in a silvermatrix. In the finished cable, these silver matrix strip conductors areapplied, e.g., stranded, to a carrier, e.g., a corrugated tube, and aresoldered at their ends for contact.

From EPO 044 144 B1 it is known to provide conductors with an insulatingceramic outside layer to insulate the filamentary superconductor wiresmade of an intermetallic compound electrically with respect to oneanother, so that they can be wound into a magnet coil. This outer layeron the wires is produced in situ by applying an aqueous suspension ofsodium silicate, kaolin and calcium carbonate, drying it andheat-treating it at a temperature in excess of 500° C.

EP 0 322 619 A1 describes the application of a film between a carriermade, e.g., of strontium titanate, or also a metal, and its coating madeof an oxide ceramic high-temperature superconducting material to ensuregood adhesion between carrier and coating and prevent chemical reactionsand interactions between carrier material and oxide ceramic material.For this interlayer, this document specifies aluminum oxide, amongothers, which is deposited on the carrier surface in the form of analcoholic suspension and is sintered at 950° C. to obtain a permanentconnection with the carrier material.

In the production process, these strip conductors are sintered in knownmanner at correspondingly high temperatures. The strip conductors, orparts of these long strip conductors (several 100 m long) are helicallywound onto an annealing base and are annealed lying next to one another.To prevent the strip conductors from sticking together or adheringduring the required annealing processes, a coating of preferablyaluminum oxide is provided on each individual strip conductor. Thismeasure prevents the individual layers/windings of this strip conductorfrom sticking together/adhering at high temperatures in excess of 800°C., at which the mechanical loadability of the silver is low.

To produce a superconductor cable, the finished strip conductors must bewound individually and bonded to corresponding terminals. For electricalbonding of each end of such a superconductor cable with terminals, thecable end and the terminal must be soldered together. For this purpose,it must be possible reliably to solder the individual strip conductorsto the terminal. This is prevented, however, by the otherwise requiredaluminum oxide coating of the strip conductors. A solution musttherefore be found to correct this incompatibility.

The object of the present invention is to define a solution that willprevent the strip conductors from sticking together during the annealingprocess and will simplify soldering for subsequent bonding of the stripconductors.

Such a solution as well as further developments and additionalembodiments are set forth in claim 1 and the dependent claims.

For the soldering process to bond the strip conductors after theproduction process of the strip conductors and their annealingprocesses, the present invention provides that the aluminum oxide usedfor coating the strip conductors, which is necessary per se, bespecifically selected. It was found that the subsequent solderingprocess is decisively facilitated if an aluminum oxide in an aqueoussuspension is used, with the finely distributed aluminum oxide particlescontained in the suspension having a particle size of no more than about0.5 μm to about 10 μm. Preferably, the suspension is deposited in such away that it (initially) results in a film thickness on the stripconductor of about 10 μm (maximum). The selected aqueous suspension isapplied to the silver matrix strip conductor, e.g., in a continuousprocess. Care should be taken that no (residues of) solvent(s), whichare, or might be, used for cleaning, remain on the surface of the stripconductor. This would increase the surface tension with respect to thesuspension to the point where a nearly closed coating with the providedsuspension is no longer possible. Coarse contamination of the silversurface should also be avoided.

According to a variant of the invention, it may also be provided,however, that the aluminum oxide coating to be produced on the conductorstrip be made so thin that it is porous. Such porosity can also serve,in particular, to contact the strip conductor material directly with thesubsequently used solder without first having to influence the appliedfilm mechanically.

For environmental reasons, it is advantageous according to the inventionto use an aqueous suspension.

The suspension described above is applied to the surface of each stripconductor and subsequently dried in a continuous annealing furnace attemperatures in excess of 100° C., e.g., at about 600° C.

It is preferred to use aluminum oxide that contains crystallizationwater in the alumina particles. With the use of the aforementioned hightemperatures, outgassing of the crystallization water occurs attemperatures in excess of 500° C., which can also lead to a certainporosity of the finished aluminum oxide coating. It also causes theadhesion characteristics of the aluminum oxide particles of the coatingon the silver surface of the strip conductor to be less strong accordingto the invention, so that the coating can subsequently, i.e. for thefollowing soldering process, but also between the individual annealingtreatments, be readily removed again at the soldering point by anultrasound treatment of several minutes.

It may be advantageous if the strip conductor, i.e. the silver, isslightly roughened prior to applying the suspension. Afterapplying/producing the coating and prior to soldering, it may beadvantageous to clean each strip conductor at the soldering point bymeans of solvents, acids or the like, particularly by means of acetone,and to apply the solder directly.

It may also be provided that after sintering, the aluminum oxide coating(which is no longer required after the sintering process) be at leastpartially removed again from the corresponding strip conductor along theconductor's entire length. This for the purpose of inspecting the stripconductor in a continuous process for its properties and the possibilityof further enhancing or equalizing its mechanical and/or electricalproperties by subsequent foreign metal diffusion into the silver in aknown process.

The flow diagram described below is intended to provide an overview ofthe inventive method. Step 1 comprises the known process of assembling,pressing, extruding, etc. the yet unsintered strip conductor into asilver matrix. The second step, after the selection of the aluminumoxide and the process parameters according to the invention, comprisesthe coating of the strip conductor with the inventive aqueous aluminumoxide suspension with a particle size according to the invention.

In the third step the suspension is dried to produce the solid coating.

The fourth step provides for one-time, but preferably multiple,sintering of the helically arranged strip conductor or strip conductorbundle.

The fifth step concerns the exposure of the strip conductor ends andcontact soldering of the strip conductors to one another and/or to theterminals at the cable end

According to a further development of the invention, during the fourthstep, i.e. after the individual annealing treatments, as well as betweenthe fourth and fifth step, an ultrasound treatment of the stripconductor ends may be optionally included as described.

What is claimed is:
 1. Method for producing high-temperaturesuperconducting silver matrix strip conductors for high-temperaturesuperconductor cables with an at least temporary aluminum oxide coating,comprising applying to the strip conductors a coating of aluminum oxidein the form of a water-based suspension of finely distributed aluminumoxide particles with a particle size of 0.5 μm to 10 μm in a filmthickness of up to 10 μm, such suspension omitting any flammablesolvents, and subsequently drying the suspension adhering to the stripconductors at a temperature in excess of 100° C.
 2. Method as claimed inclaim 1, in which the drying is carried out at approximately 600° C. 3.Method as claimed in claim 1, in which prior to applying the suspension,the strip conductors to be coated are cleaned while avoiding the use ofacetonic or alcoholic solvents.
 4. Method as claimed in claim 1, inwhich prior to applying the suspension, the strip conductors areroughened.
 5. Method as claimed in claim 1, further comprising cleaningwith a solvent, at soldering points, the coated strip conductors. 6.Method as claimed in claim 5, in which the solvent used is acetone. 7.Method as claimed in claim 1, further comprising cleaning with an acid,at soldering points, the coated strip conductors.
 8. Method as claimedin claim 1, in which the aluminum oxide comprises water of constitutionand alumina particles, with the water of constitution being outgassedfrom the coating when drying at temperatures in excess of 500° C. 9.Method as claimed in claim 1, in which the coating on the stripconductors is sintered when drying at temperatures of up to 900° C. 10.Method as claimed claim 1, in which the coating on the strip conductorsis sintered when drying at temperatures of up to 820° C.
 11. Method asclaimed in claim 1, further comprising using ultrasound to remove thecoating from a part of at least one of the strip conductors.
 12. Methodas claimed in claim 11, further comprising using ultrasound to removethe coating at defined soldering points of at least one of the stripconductors.
 13. Method as claimed in claim 1, further comprisingremoving the coating from the entire strip conductors after the dryingstep.
 14. Method as claimed in claim 13, in which, after removing thecoating, foreign metals are diffused into the strip conductors.